Enumerate the assumptions that we undertake in making the respiratory balance sheet. Are these assumptions valid for a living system? Compare fermentation and aerobic respiration in this context.

(N/A) It is possible to make a calculation of the net gain of $ATP$ for every glucose molecule oxidized, but in reality, this remains a theoretical exercise. These calculations are based on the following assumptions:
$(1)$ There is a sequential, orderly pathway functioning, with one substrate forming the next, and with glycolysis, $TCA$ cycle, and $ETS$ pathway following one after another.
$(2)$ The $NADH$ synthesized in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
$(3)$ None of the intermediates in the pathway are utilized to synthesize any other compound.
$(4)$ Only glucose is being respired; no other alternative substrates are entering the pathway at any of the intermediary stages.
These assumptions are not valid in a living system because:
- All pathways work simultaneously and do not take place one after another.
- Substrates enter the pathways and are withdrawn as and when necessary.
- $ATP$ is utilized as and when needed.
- Enzymatic rates are controlled by multiple means.
Comparison between Fermentation and Aerobic Respiration:

Fermentation	Aerobic Respiration
$(1)$ Glucose is partially decomposed into ethanol or lactic acid.	$(1)$ Complete decomposition takes place, forming $CO_{2}$ and $H_{2}O$.
$(2)$ Only two net molecules of $ATP$ are obtained.	$(2)$ $A$ large number of $ATP$ molecules are formed.
$(3)$ Oxidation of $NADH$ to $NAD^{+}$ is a slow reaction.	$(3)$ Oxidation of $NADH$ to $NAD^{+}$ is a fast reaction.